You can verify if that's the request that blocks by using 'telnet api.domain.tld 80' or 'openssl s_client -crlf api.domain.tld:443' and copy-paste that address with an extra enter at the end. If you get a result immediately, it's not the blocking request. After the final enter (you should have an empty line at the end of your input), there should not be a response for several seconds (depending of your TimeForCGI setting).